1. Field of the Invention
The present invention relates generally to conveyors for aggregate material and, more particularly but without limitation, to conveyors for conveying hot mix asphalt material in an asphalt paving machine.
2. Description of the related art
Hot mix asphalt material used in highway construction and the like ideally consists of a uniform mixture of several sizes of mineral aggregate and liquid bituminous asphalt cement. When properly blended in the correct proportions, the hot mix asphalt material provides a uniform and durable material that is capable of withstanding heavy traffic and loads over a long service life. In the process of producing the hot mix asphalt material and delivering it to a construction site for placement by an asphalt paver, however, the mixture may tend to separate into its constituent parts, a condition commonly referred to in the industry as "segregation".
For instance, the hot mix asphalt material produced at the production plant is often loaded into a storage silo to await trucks for transport to the construction site. As the hot mix asphalt material is carried by a conveyor to the top of the silo, the larger sized aggregate contained in the hot mix asphalt material tends to separate from the smaller sized aggregate material contained therein. Further, as the hot mix asphalt material is dropped from the conveyor into the silo, the larger sized aggregate tends to roll to the lower periphery of the pile of hot mix asphalt material typically forming a pyramid near the center of the silo, while the smaller sized aggregate tends to cling to the top and sides of the pile. The hot mix asphalt material within the silo is, therefore, no longer a uniform mixture as desired but, instead, is a segregated mixture consisting of a surplus of larger sized aggregate near the outer wall of the silo and a paucity of larger sized aggregate nearer the center of the silo.
Similarly, as the hot mix asphalt material is discharged from the silo into trucks for transport, larger sized aggregate tends to roll to the extreme corners of the truck box while the smaller sized aggregate tends to remain more toward the center of the truck box. The truck therefore contains a segregated mixture consisting of a surplus of larger sized aggregate at the front, rear and sides of the truck box and a paucity of larger sized aggregate nearer the center of the truck box.
An asphalt paver at the construction site is generally a state-of-the-art self-propelled construction machine designed to receive, convey, distribute, profile and partially compact the hot mix asphalt material. The paver accepts the hot mix asphalt material into a receiving hopper at the front of the machine, conveys the material from the hopper to the rear of the machine with parallel slat conveyors, distributes the hot mix asphalt material along the width of an intended ribbon or mat by means of two opposing screw or spreading conveyors, and profiles and compacts the hot mix asphalt material into a mat with a free-floating screed.
Each slat conveyor that moves the hot mix asphalt material from the receiving hopper to the rear of the paving machine generally consists of two parallel slat chains with a multitude of transverse slats connected there between. Each slat chain is pulled by one of two sprockets mounted on a common shaft which, in turn, is driven by appropriate power transmission chains, gear boxes or the like. Because the slat conveyor pulls the hot mix asphalt material from the hopper in a bulk mass with little or no remixing, any segregated characteristics of the hot mix asphalt material as it exists in the hopper continues to exist in the hot mix asphalt material as it is placed by the opposing screw conveyors on the subgrade in front of the screed.
Thus, when the undesirably segregated, hot mix asphalt material is delivered to the construction site and placed by the asphalt paver, the mat produced is not of uniform consistency but, instead, contains regions having a surplus of larger sized aggregate and a paucity of smaller sized aggregate and, likewise, regions with a surplus of smaller sized aggregate and a paucity of larger sized aggregate. As a result, the mat produced from the segregated material does not possess the desired mechanical properties and generally will not withstand the anticipated loads and stresses as well as a mat constructed of non-segregated, or uniform, hot mix asphalt material. What is needed is an apparatus that is capable of, and a method for, remixing segregated hot mix asphalt material just before the hot mix asphalt material is placed on a subgrade by an asphalt paver whereby the detrimental effects of segregation are substantially or totally eliminated.